1. Field of the Invention
This invention relates to a webbed multiple sheet having at least one center layer arranged between two parallel, substantially plane polymethylmethacrylate (PMMA) outer layers, wherein the outer and center layers are bonded into one piece by webs.
2. Description of the Prior Art
Webbed multiple sheets which exhibit at least one center layer arranged between two parallel, substantially plane PMMA outer layers and which are bonded into one piece by webs are known and are commercially available. They are produced by extrusion with simultaneous formation of the outer and center layers and of the webs.
After the hollow extruded material leaves the extrusion die, the outer layers are cooled below the softening temperature while passing through a vacuum forming channel. The center layers and the webs remain thermoplastic longer than the outer layers. Since they are not supported, the center layers tend to sag between the webs in case of horizontal extrusion. This effect can be reduced if the center layers are made very thin because in that case, less heat must be performed in order to get below the softening temperature and if, in addition, the extruding is performed very slowly. Hence, it is desirable to maintain a minimum thickness for the center layers. However, even very thin center layers are still damaged to some degree.
Therefore, a need continues to exist for a webbed multiple sheet having a center layer with a reduced tendency to sag or break.